1. Field of the Invention
The present invention generally relates to an optical amplifier, and, more particularly, it relates to a Raman amplifier.
2. Description of the Related Art
In wavelength division multiplexed (WDM) systems used in current optical communication systems, as methods for increasing transmission capacity, there are a method for expanding a signal wavelength band to increase the number of multiplexed wavelengths, and a method for enhancing a transmission rate (bit rate) per wavelength. In normal WDM systems, due to limitation in gain wavelength band of an Erbium doped fiber amplifier (EDFA), most generally, a wavelength for optical signal is selected from a wavelength band (called as a C-band) of about 1530 to 1565 nm. On the other hand, since optical amplification can be effected by the EDFA also in a wavelength band (called as an L-band) of about 1575 to 1610 nm, recently, WDM systems regarding this band has been developed.
By expanding the wavelength band as mentioned above, capacity which can be transmitted by a single WDM system can be increased. Regarding the WDM systems, since a C-band WDM system has firstly been developed, in order to increase the transmission capacity of the WDM system, it is desirable that the existing equipment for the C-band WDM system is utilized and an L-band WDM system is added to the equipment to gradually increase the capacity. In the conventional WDM systems using the EDFA, the transmission rate per wavelength has been enhanced (speeded up) by technically improving various elements constituting a transmission system. However, in systems utilizing a discrete amplifier such as EDFA, achievement of higher speed has been approaching to its limit. In order to achieve further high speed and/or longer distance transmission, it is said that incorporation of a distributed amplifier such as a Raman amplifier into the system is inevitable, and, to this end, various developments have been made vigorously to permit practical use.
As shown in FIG. 20, the Raman amplifier comprises an optical fiber as an amplifying medium, and a pump source for generating stimulated Raman scattering in the fiber. When silica-based optical fiber is used as the amplifying medium, peak of gain is generated at a longer wavelength side than a wavelength of the pump light, i.e., in a frequency band smaller than that of the pump light by about 13.2 THz. For example, since a difference in wavelength between 1450 nm and 1550 nm corresponds to 13.2 THz, a pump light having a wavelength of about 1450 nm is used in order to amplify the C-band, and a pump light having a wavelength of about 1490 nm is used in order to amplify the L-band (FIG. 21).
However, single wavelength pumped Raman gain has great wavelength dependency, and as apparent from FIG. 22, from when the Raman gain exceeds about 5 dB, it is impossible to suppress gain deviation below 1 dB regarding an operating band width of 30 nm. In order to solve this problem, it is effective that a plurality of pump lights having proper wavelength interval are applied to the Raman amplifier (i.e., the amplifier is pumped by a multi-wavelength pump source). According to this method, Raman amplification having better gain flatness can be achieved in a wider band than the conventional band. As disclosed in Japanese Patent Publication No. 7-99787 (1995) (particularly, in FIG. 4 thereof), such concept itself can be understood intuitively. Japanese Patent Application Nos. 10-208450 (1998) and 11-34833 (1999) refer concrete values of the wavelength interval and assert that the proper value is 6 nm to 35 nm.
FIGS. 23A and 23B show examples of Raman gain profiles obtained when the pump light intervals are selected to 4.5 THz and 5 THz, respectively and DSF is used as an amplifying fiber. As apparent from FIGS. 23A and 23B, when the pump light interval is increased, a valley of gain is deepened and gain deviation is increased. In FIG. 23A, values shown in the following Table 1 were used as frequency (wavelength) of the pump light and, in FIG. 23B, values shown in the following Table 2 were used as frequency (wavelength) of the pump light. In this case, the pump light interval of 4.5 THz corresponds to 33 nm and the pump light interval of 5 THz corresponds to 36.6 nm. That is to say, these examples show the fact that the gain flatness is not so good if the pump light interval becomes more than 35 nm.
FIG. 24 shows a gain profile obtained when the pump light interval is selected to 4.5 THz and three wavelengths are used. From FIG. 24, it can be seen that, when the third pumping wavelength is added, the valley of gain is deepened in case of the pump light interval of 4.5 THz. FIG. 25 shows a gain profile obtained when the pump light intervals are selected to 2.5 THz and 4.5 THz, respectively and three wavelengths are used. In comparison with FIG. 24, the valley of gain becomes shallower. Since the frequency interval of 2.5 THz used here corresponds to the wavelength interval of about 18 nm, also in this case, the wavelength interval is included in the range from 6 nm to 35 nm. However, considering conversely, it can be said that, even when the wavelength interval is included in the range, if the wavelength interval is not set properly, flat gain cannot be obtained.
By the way, in designing the conventional WDM optical amplifiers, the object was to reduce the gain flatness as small as possible, and an optical amplifier in which all of optical signals are subjected to the same gain was ideal. When the number, power and band width of the used signals are small, such design concept is adequate. However, as the used band of the optical signal is increased, there arose a problem regarding Raman amplifying effect between optical signals. As disclosed in journal (for example, S. Bigo et al, xe2x80x9cIEEE Photonics Technology Lettersxe2x80x9d, pp. 671-673, 1999), in this phenomenon, WDM signals which were set to have same powers upon incident on a transmission line tend to include linear tilt in which power becomes small at a short wavelength side and great as a long wavelength side after transmission. Such tilt is determined by various factors such as the number, power and band width of the optical signals, property of a fiber constituting the transmission line and a transmission distance. As means for coping with this problem, there has been proposed an tilt compensater (T. Naito OAA""99, WC5) for attenuating the long wavelength side signal by using a loss medium having wavelength dependency and a method (M. Takeda et al, OAA""99, ThA3) for compensating tilt by give relatively great gain to the short wavelength side signal by using wavelength dependency of Raman gain. Since the former method for giving the loss has disadvantage due to noise, the later method is more excellent. However, in the paper written by Takeda et al, since the tilt of Raman gain is not linear, the gain flatness after compensating the tilt is relatively great (more than 1 dB).
Similar to the WDM system using only the above-mentioned EDFA, also in the WDM system using the Raman amplifier, when the WDM system for C-band is introduced, it is desirable that the system is designed so that the WDM system for L-band can be added while maintaining the function of the equipment.
In a Raman amplifiers using wavelength multiplexed pumping, when it is desired to expand the gain band, like band extension from C band to C+L band, it is necessary that, while utilizing all of pumping wavelengths which were used before expansion, after the expansion, the amplifier can be operated for the C+L band. That is to say, it must be designed so that, by adding new pumping wavelength to the pumping wavelength used for the C-band, the amplifier can be operated for the C+L band. In this case, it is necessary that wavelength arrangement for flattening the gain in the C-band and the C+L band can be commonly used.
Since the gain deviation is proportional to the magnitude of the peak gain, if the gain is great, the pump light interval must be set small. Further, as mentioned above, in a case where the pump lights are equidistantly arranged, even when the wavelength interval is smaller than 35 nm, the gain deviation may not be reduced sufficiently. Also in this case, it is necessary to use narrower wavelength interval. Although the gain deviation can be reduced by reducing the pump light interval in principle, due to problems regarding a wave combining technique and cost, practically, the pump light interval has a lower limit. In Japanese Patent Application Nos. 10-208450 (1998) and 11-34833 (1999), the lower limit is determined to 6 nm on the basis of the wave combining technique.
However, in the above-mentioned Japanese Patent Application Nos. 10-208450 (1998) and 11-34833 (1999), although the fact that the interval between two adjacent pumping wavelengths is preferably within a range from 6 nm to 35 nm is disclosed, adequate information regarding detailed design values is not disclosed. Further, in the design described in a published paper (Y. Emori et al, OFC""99 PD19), the gain deviation is 1 dB, and this technique cannot be applied when smaller gain flatness is required.
In the present invention, a method for selecting wavelength disposition in a Raman amplifier using three or more pumping wavelengths is disclosed, and a primary object of the present invention is to provide a Raman amplifier having good gain flatness, and further, an object of the present invention is to provide a Raman amplifier in which gain deviation becomes about 0.1 dB for a peak value of Raman gain of 10 dB.
Further, another object of the present invention is to provide a Raman amplifier suitable for compensating Raman effect between signals which arises a problem during wide band WDM transmission.
In the present invention, it is also considered to provide a Raman amplifier in which, when a gain band is expanded by adding new pump light, gain and gain flatness after expansion are not deteriorated considerably in comparison with before expansion.
The Inventors investigated the gain profiles of Raman amplifiers utilizing wavelength multiplexed pumping and analyzed wavelength disposition for flattening the gain. The principle is as follows.
The gain profile of the Raman amplifier utilizing wavelength multiplexed pumping is obtained by the superposition of gains generated by respective pumping wavelengths. Accordingly, a combination of gain tilts which are cancelled with each other is one of factors for obtaining flat gain. That is to say, a gain property having good wavelength flatness can be obtained by combining a rightwardly and downwardly extending curve (negative gain tilt) in which the gain is decreased from the short wavelength side toward the long wavelength side and a rightwardly and upwardly extending curve (positive gain tilt) in which the gain is increased from the short wavelength side toward the long wavelength side. When the number of pumping wavelengths is two, the tilt at a longer wavelength side from gain peak of a gain curve obtained by the shorter wavelength pump is combined with the tilt at a shorter wavelength side from gain peak of a gain curve obtained by the longer wavelength pump. As apparent from FIG. 21, the Raman gain curve of one wavelength pumping which is a basic element for superposition has two gain peaks rather than one, and, in the C-band, the first peak at the end of positive gain tilt is located at 1550 nm and the second peak at the end of negative gain tilt is located at 1560 nm. Further, in the L-band, the first peak at the end of positive gain tilt is located at 1595 nm and the second peak at the end of negative gain tilt is located at 1605 nm. Thus, in any cases, two pumping wavelengths must be spaced apart from each other by 10 nm or more. In the curve A shown in FIG. 21, the center wavelength of the pumping wavelength is 1450 nm, and, in the curve B, the center wavelength of the pumping wavelength is 1490 nm.
FIG. 26 shows an example of the gain profile of the Raman amplifier designed for the C-band and the following Table 3 shows pumping wavelengths used therefor. A supposed fiber is a normal single mode fiber, and the gain band is designed to cover 1530 nm to 1565 nm. The gain profile of the Raman amplifier utilizing the wavelength multiplexed pumping is obtained by the superposition of gains generated by respective pumping wavelengths. In FIG. 26, the allocation of gain magnitude caused by each pump wavelength is optimized to minimize the flatness of the gain obtained by addition. The number of pumping wavelengths is appropriately selected in accordance with the desired gain flatness. Similarly, FIG. 27 shows an example of the gain profile of the Raman amplifier designed for the L-band and the following Table 4 shows pumping wavelengths used therefor.
When the number of pumping wavelengths is two, the gain band width can be widened by increasing the interval between the wavelengths. However, if the interval is too wide, the valley of gain will be created in the band. Accordingly, the gain flatness and the gain band have a relationship of xe2x80x9ctrade-off.xe2x80x9d In FIGS. 26 and 27, the wavelength intervals of pump lights are determined to obtain the optimized results in pre-determined gain bands, and such intervals are 27 nm and 29 nm, respectively (refer to the Tables 3 and 4).
In the Raman gain curve of one wavelength pumping which is a basic element for superposition, as shown in FIG. 21, the gain tilt at the longer wavelength side from the gain peak is more steep than the gain tilt at the shorter wavelength, and the band in which the tilt can be utilized is narrow. In order to widen the band by using a more gentle negative gain tilt, the negative gain tilt must be formed by using a plurality of pumping wavelengths.
When the gain curve having the negative gain tilt is formed by using three or more pumping wavelengths, similar to two pumping wavelengths, the pumping wavelengths for forming the negative gain tilt must be spaced apart from the pumping wavelengths for forming the positive gain tilt by 10 nm or more. However, since the pump light for forming the negative gain tilt is constituted by the plurality of wavelengths, the longest pumping wavelength among them is spaced apart from the pumping wavelength for forming the positive gain tilt by 10 nm or more. This corresponds to the interval between 1435 nm and 1460 nm in three (3) wavelengths pumping and the interval between 1438 nm and 1462 nm in four (4) wavelengths pumping in the Table 3. Further, this corresponds to the interval between 1475 nm and 1500 nm in three (3) wavelengths pumping and the interval between 1478 nm and 1501 nm in four (4) wavelengths pumping in the Table 4.
In a case where three or more pumping wavelengths are used, when the intervals between the pumping wavelengths is approximately equidistant, ripple in the negative gain tilt generated by the combination becomes small. When the flatness is obtained by combination with the positive gain tilt, such ripple determines the final gain flatness. It is demonstrated by the four wavelength pumping case in FIG. 26 and FIG. 27. As shown in the Tables 3 and 4, the optimized wavelengths are 1423 nm, 1430 nm, 1438 nm and 1462 nm, 1470 nm, 1478 nm, which provide approximately equidistant disposition.
FIGS. 28 to 30 show the performance of the Raman gain curves when the pump light intervals are equidistant. FIG. 28 shows an example that the peak gain is adjusted to 10 dB under a condition that the gains generated by each pump lights are the same. From FIG. 28, it can be seen that the narrower the pump light interval is, the smaller the unevenness of the gain is. FIG. 29 shows an example that the gains obtained by the respective pump lights are adjusted to flatten the gain. Also in this case, similar to FIG. 28, the narrower the pump light interval is, the smaller the unevenness of the gain is. Further, it can be seen that undulation of the gain curve in FIG. 28 determines the maximum gain deviation in FIG. 29. Thus, in order to bring the gain deviation to about 0.1 dB, the pump light interval of 2 THz is too wide, but 1 THz is adequate.
FIG. 30 shows the pattern when the multiplexed amount in number is changed while maintaining the pump light interval in 1 THz. As can be seen from a gain curve of 1 ch pumping, in case of a silica-based fiber, a smooth curve having no unevenness is obtained at the shorter wavelength side from the gain peak, however, there are three relatively prominent unevenness at the longer wavelength side, which is a factor for determining the limitation of the flatness. The unevenness is decreased as the multiplexed amount in number is increased. For example, referring to the gain curve of 1 ch, although this curve has protrusion of about 1 dB in the vicinity of 187 THz, as the multiplexed amount in number is increased, such protrusion gradually becomes smaller. The reason is that, since the peak gains are set to be the same, as the multiplexed amount in number is increased, the gain per one wave is decreased to reduce the magnitude of the protrusion itself and the unevenness of the same shape (concavities and convexities) are added with slight equidistant intervals. That is to say, the convexities of the gain curve of a certain pumping wavelength are added to the concavities of the gain curve of another pumping wavelength to reduce the entire unevenness. The value xe2x80x9cabout 1 THzxe2x80x9d specified in claims 2 to 4 is based on this principle and is also based on the fact that frequency difference between the protrusion in the vicinity of 187 THz and the immediately adjacent recess in the vicinity of 188 THz is about 1 THz in the gain curve of 1 ch pumping shown in FIG. 30. Accordingly, depending upon the fiber used, the gain curve of 1 ch pumping may be slightly changed and, thus, the value specified in claims 2 to 4 as xe2x80x9cabout 1 THzxe2x80x9d may also be changed. In any case, in order to reduce the gain deviation, the unevenness of the gain curves to be added to each other must be cancelled with each other.
Since the limit of the gain deviation is determined by the undulation and unevenness of the each gain curves to be overlapped or combined, it is considered that a flat gain profile having small gain deviation can be obtained by combining the gain curves having small unevenness. Accordingly, this can be achieved by combining the gain curve generated by the pump light multiplexed with interval of about 1 THz with the gain curve generated by the pump light located at the longer wavelength side than the former pump light. In this case, in view of expansion of the band, it is desirable that the peaks of two gain curves are spaced apart from each other moderately.
While the effects as mentioned above was explained for the purpose of reducing the gain flatness, by reducing the gain of the pump light at the long wavelength side, a gain profile in which gain is linearly reduced from the short wavelength side toward the long wavelength side can be achieved. When this is combined with the tilt of optical signal level created by the Raman effect between the optical signals, the level of the optical signal can be flattened. Since any tilt can be realized by adjusting the distribution of gain between the short wavelength side and the long wavelength side, any Raman tilt can be compensated for.
When the gain bands of the C-band and L-band are tried to be expanded, it is considered that simultaneous use of the pumping wavelength for C-band and pumping wavelength for L-band is optimum.
However, when both the pump lights for C-band and L-band shown in FIGS. 26 and 27 are simultaneously used, gain profiles such as shown in FIG. 31 are obtained even if the gain allocation is optimized so as to be flatten. The following Table 5 shows pumping wavelengths used in this case. In this case, if the gain flatness is tried to be equal to those shown in FIGS. 26 and 27 in the entire area of C+L band, as shown in FIG. 31, a big recess is generated in the C-band, thereby more worsening the gain flatness than before expansion.
For the same reasons as mentioned above, also when the operation of good gain flatness is effected in the C+L band, it is necessary that the negative gain tilt is formed by using the plurality of equidistant pumping wavelengths and the positive gain tilt is formed by using the pump light located at the longer wavelength side from the longest pumping wavelength by 10 nm or more. However, since the pumping wavelengths used in FIG. 31 are obtained only by simultaneously using the pumping wavelengths of FIGS. 26 and 27, the requirement xe2x80x9capproximately equidistant dispositionxe2x80x9d cannot be satisfied in the pumping wavelength band for C-band. The Table 5 shows the pumping wavelengths used in FIG. 31. According to this, in order to obtain the approximately equidistant disposition at the short wavelength side, it can be seen that the pumping wavelength is insufficient in the pumping wavelength band for C-band. It was found that such insufficient amount becomes a factor for generating the recess in gain shown in FIG. 31.
From the above results, it was analyzed that means required for achieving the above object are as follows.
According to a means of the present invention, in a Raman amplifier using three or more pumping wavelengths, when the pumping wavelengths are divided into a short wavelength side group and a long wavelength side group at the boundary of the pumping wavelength having the longest interval between the adjacent wavelengths, the short wavelength side group includes two or more pumping wavelengths having intervals therebetween which are substantially equidistant, and the long wavelength side group is constituted by two or less pumping wavelengths.
According to another means of the present invention, when a shortest pumping wavelength is defined as a first channel and pumping wavelengths which are spaced apart from each other by about 1 THz from the shortest pumping wavelength toward a long wavelength side are defined as second to n-th channels, respectively, pump lights having wavelengths corresponding to the first to n-th channels are multiplexed, and pump light having a wavelength spaced apart from the n-th channel by 2 THz or more toward the long wavelength side is further combined, and resultant pump light is used as pump light for a Raman amplifier. Further, when the shortest pumping wavelength is defined as the first channel and pumping wavelengths which are spaced apart from each other by about 1 THz from the shortest pumping wavelength toward the long wavelength side are defined as the second to n-th channels, respectively, all of the wavelengths corresponding to the channels other than (nxe2x88x921)th and (nxe2x88x922)th channels are combined with each other, resultant pump light is used for the Raman amplifier. Alternatively, all of the wavelengths corresponding to the channels other than (nxe2x88x922)th and (nxe2x88x923)th channels are combined with each other, resultant pump light is used for the Raman amplifier.
According to still another means of the present invention, in a Raman amplifier for expanding a gain wavelength band, there are provided two or more pumping wavelengths before expansion, and two or more pumping wavelengths are added for expanding the gain wavelength band, at least one of the pumping wavelengths to be added is differentiated from the pumping wavelengths used before expansion, and at least one of the differentiated pumping wavelengths is positioned within the pumping wavelengths bands used before expansion.
According to still another means of the present invention, in a Raman amplifier for expanding a gain wavelength band, there are provided two or more pumping wavelengths before expansion, and two or more pumping wavelengths are added for expanding the gain wavelength band, at least one of the pumping wavelengths to be added is differentiated from the pumping wavelengths used before expansion, and at least one of the differentiated pumping wavelengths is positioned within a pumping wavelength band having insufficient gain among the pumping wavelengths bands used before expansion.
According to still another means of the present invention, in a Raman amplifier for expanding a gain wavelength band, there are provided two or more pumping wavelengths before expansion, and one or more pumping wavelength is added to the pumping wavelengths bands before expansion so that, by the addition, the pumping wavelengths within the pumping wavelengths bands before expansion are spaced apart from each other equidistantly or substantially equidistantly.
According to a Raman amplifier of the present invention, when a C-band and an L-band are simultaneously amplified by simultaneously using two or more pumping wavelengths for amplifying the C-band and two or more pumping wavelengths for amplifying the L-band, one or more pumping wavelength different from the pumping wavelengths for the C-band used before expansion is added to bands of the pumping wavelengths for the C-band.
According to another Raman amplifier of the present invention, when a C-band and an L-band are simultaneously amplified by simultaneously using two or more pumping wavelengths for amplifying the C-band and two or more pumping wavelengths for amplifying the L-band, one or more pumping wavelength different from the pumping wavelengths for the C-band used before expansion is added to a band of a wavelength having insufficient gain among bands of the pumping wavelengths for the C-band.
According to still another Raman amplifier of the present invention, when a C-band and an L-band are simultaneously amplified by simultaneously using two or more pumping wavelengths for amplifying the C-band and two or more pumping wavelengths for amplifying the L-band, one or more pumping wavelength different from the pumping wavelengths for the C-band used before expansion is added to bands of the pumping wavelengths for the C-band so that, by the addition, the pumping wavelengths within the bands of the pumping wavelengths before expansion are spaced apart from each other equidistantly or substantially equidistantly.